Despite work by many investigators, we still do not know the functional role of medial olivocochlear (MOC) efferents. In previous attempts to correlate MOC strength with subject performance, the passive response of the MOC acoustic reflex was measured, not the MOC activation during the task. We are now able to measure MOC activation during a psychophysical task, a new paradigm that greatly increases our ability to test MOC function and descending control in the human auditory system. MOC tests based on otoacoustic emission (OAE) measurements are being used both scientifically and clinically, but the accuracy of such tests has never been ascertained. To maximize the scientific and clinical usefulness of OAE tests we will: Aim (1A,B) Measure MOC effects using the three types of evoked OAEs and compare the resulting MOC-strength metrics in terms of signal/noise ratios and accuracy of subject ranking, using both contralateral and ipsilateral elicitors, and Aim (1C) determine, in humans, MOC reflex strength in terms of changes in cochlear sensitivity, as shown by MOC-induced changes in wave I of the auditory brainstem response. To address the functional significance of the MOC system, we will systematically measure MOC activation levels in psychophysical tasks: (Aim 2A-C) in which MOC activation is expected to aid performance (discrimination of transient signals in noise) vs. those in which no benefit is anticipated, and (Aim 2D-E) to determine MOC frequency selectivity in an active task. The MOC strength tests developed in this work should be useful in clinical and scientific contexts to predict susceptibility to noise damage, in the diagnosis and tracking of diseases, in evaluating the toxicity of drugs and to show the role of MOC feedback in normal hearing. The proposed work will also help to indicate the extent to which such feedback should be put in hearing aids and prosthetic devices such as cochlear implants.